The bromination of 3-arylpropynes: Evidence for phenyl participation at a vinyl cation

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Abstract
The bromination of substituted 3-arylpropynes in acetic acid has been examined. The only products obtained are 1,2-dibromo-3-arylpropenes; the stereochemistry of the dibromides is predominantly trans except for the 4-methoxy isomer which gives 83% cis and 17% trans. In the presence of 0.1 M lithium bromide all substrates give predominantly trans products. Combined with rate studies, these results indicate that these acetylenes react by an unsaturated cyclic bromonium ion except for the 4-methoxy case where phenonium ion stabilization of a vinyl cation is implicated. In the presence of lithium bromide a bromide ion catalyzed addition of molecular bromine (AdE3) occurs.

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